EXECUTIVE SUMMARY

The goal of this worklog is to allow the control over whether or not the replica threads start upon server restart to be extended to MySQL users and MySQL privilege infra-structure. At the moment, only operating system privileged users may add --skip-slave-start to control it, as a parameter and at server bootstrap.

More specifically, the worklog will transform the --skip-slave-start command line parameter into skip_slave_start, a global, persistable, read-only system variable.

USER STORIES

As a DBA, need to control whether or not the replica applier starts upon server restart, without having the necessary privileges or access to the operating system infra-structure needed to pass --skip-slave-start upon server start and without having to clear replication channel configurations. Will achieve it by using SET PERSIST_ONLY with the skip_slave_start global variable.

As a DBA, need to understand if the replica applier started when the server was started in order to debug or troubleshoot issues with my running instance. Will achieve it by consulting the value of the skip_slave_start global variable.

As an external tool configuring InnoDB ClusterSets, need to prevent replication channels to start automatically upon server start when: GR fails to initiate upon server restart, after a primary election where the server became a secondary; group_replication_start_on_boot is disabled. In either case, it's needed by external tools (Shell/AdminAPI) to: query the state of skip_slave_start; change the value of skip_slave_start to prevent replication applier to start when GR isn't started, blocking the proper configuration to be triggered.

IDENTIFIED REQUIREMENTS

R1. Variable skip_slave_start shall exist as a global, read-only, persistable variable.

R2. All forms of --skip-slave-start command line parameter that are allowed before the worklog, shall be allowed after the worklog.

PROPOSED APPROACH

Re-using the current already in-place infra-structure around the --skip-slave-start parameter (i.e., internal global variable), a read-only and persist-only variable will be added using the sys_var API.

Name and description are kept, as is the ability to set the variable through the command line.

IMPACT ANALYSIS

User Interface

skip_slave_start system variable

• NAME: skip_slave_start
• VALUES: ON, OFF, TRUE, FALSE, 1, 0
• DEFAULT: OFF
• SCOPE: GLOBAL
• REPLICATED: No
• DYNAMIC: No, read-only
• PERSIST: PERSIST_ONLY
• PRIVILEGES: none to read, SYSTEM_VARIABLE_ADMIN and PERSIST_RO_VARIABLES_ADMIN to write
• COMMAND LINE: Yes
• DESCRIPTION: "If set, slave is not autostarted."
• OBSERVABILITY: as a user, execute SELECT @@skip_slave_status to access the current value of the variable

Performance

No impact in performance is expected.

Storage engines

No special concerns storage engines.

Security context

No special concerns regarding security since the already existent infra-structure and privileges used to protected the changes of a read-only persistable-only global variable will be applied to skip_slave_start, more specifically, SYSTEM_VARIABLES_ADMIN and PERSIST_RO_VARIABLES_ADMIN are needed to persist a read-only global static variable.

Upgrade/Downgrade

No special concerns regarding upgrade/downgrade.

Cross-version replication

No special concerns regarding cross-version replication since variable changes aren't replicated.

Observability

The variable state will be available and observable through the following SQL statements:

1. SELECT @@skip_slave_start
2. SELECT variable_value FROM performance_schema.global_variables WHERE variable_name = 'skip_slave_start'
3. SELECT variable_value FROM performance_schema.persisted_variables WHERE variable_name = 'skip_slave_start' () () Only available if SET PERSIST_ONLY has been executed for skip_slave_start

Deployment / installation

No changes to deployment or installation processes.

Protocol

No foreseeable changes to the MySQL protocol.

API AND DATA STRUCTURES

Changes in the current API

Prior to this work, skip_slave_start was a command line parameter, with an associated global internal variable that stores the current parameter value.

Such infra-structure will be kept and the implementation will deal only with moving the variable from the command line long-option infra-structure to the sys_var infra-structure:

  diff --git a/sql/mysqld.cc b/sql/mysqld.cc
  index e06e52e29751e029614b3f72b97c7af8e8bf2aa6..88e9117a1f1b4955085da065ef830f65da7f48a9 100644
  --- a/sql/mysqld.cc
  +++ b/sql/mysqld.cc
  @@ -8387,9 +8387,6 @@ struct my_option my_long_options[] = {
       {"skip-new", OPT_SKIP_NEW, "Don't use new, possibly wrong routines.",
        nullptr, nullptr, nullptr, GET_NO_ARG, NO_ARG, 0, 0, 0, nullptr, 0,
        nullptr},
  -    {"skip-slave-start", 0, "If set, slave is not autostarted.",
  -     &opt_skip_slave_start, &opt_skip_slave_start, nullptr, GET_BOOL, NO_ARG, 0,
  -     0, 0, nullptr, 0, nullptr},
       {"skip-stack-trace", OPT_SKIP_STACK_TRACE,
        "Don't print a stack trace on failure.", nullptr, nullptr, nullptr,
        GET_NO_ARG, NO_ARG, 0, 0, 0, nullptr, 0, nullptr},
  diff --git a/sql/sys_vars.cc b/sql/sys_vars.cc
  index 242e649f34cfdb7edd26f446d8b32cb9c3385b96..93a7dc8301c6e52f13d971b99fe1b82e44fb894d 100644
  --- a/sql/sys_vars.cc
  +++ b/sql/sys_vars.cc
  @@ -7060,3 +7060,9 @@ static Sys_var_bool Sys_replication_sender_observe_commit_only(
       GLOBAL_VAR(opt_replication_sender_observe_commit_only), CMD_LINE(OPT_ARG),
       DEFAULT(false), NO_MUTEX_GUARD, NOT_IN_BINLOG, ON_CHECK(nullptr),
       ON_UPDATE(nullptr));
  +
  +static Sys_var_bool Sys_skip_slave_start(
  +    "skip_slave_start", "If set, slave is not autostarted.",
  +    READ_ONLY GLOBAL_VAR(opt_skip_slave_start), CMD_LINE(OPT_ARG),
  +    DEFAULT(false), NO_MUTEX_GUARD, NOT_IN_BINLOG, ON_CHECK(nullptr),
  +    ON_UPDATE(nullptr));

IMPLEMENTATION PLAN

1. Step#1: move the variable from the long-options to the system variable infra-structure; add the proper test script to the sys_vars suite.